Prior art workers have devised numerous systems for liquefying substantially 100% of liquid natural gas fed into the systems. While the prior art systems work well, they are generally characterized by high expense and complexity. The present invention provides both a method and an apparatus for a greatly simplified system for liquefying substantially 100% of natural gas caused to enter the system.
In its simplest form the system comprises a source of natural gas. In addition, an engine in the form of any appropriate prime mover, a compressor and an expander are provided. These last mentioned elements are drivingly connected, as will be evident hereinafter. In addition, the system comprises at least one cooler, at least one heat exchanger, a restrictor and a liquid natural gas collector having a vent return back to the compressor. Pressurized natural gas, from an appropriate source, is fed to the inlet of the compressor. The compressed natural gas exits the compressor and passes through at least one cooler to remove the heat of compression. Thereafter, the compressed and cooled gas is split into first and second flow portions. The first flow portion passes through at least one heat exchanger so as to lower its temperature. Thereafter, it passes through the restrictor into the liquid natural gas collector. The pressure of the compressed natural gas and the performance levels of the apparatus components are such that a substantial portion of the first flow portion flashes to liquid natural gas in the collector. As will be set forth hereinafter, there will generally be some saturated vapor which will be driven out of the collector. For this reason, the collector is provided with an exhaust which is operatively connected to the compressor inlet.
The second flow portion of the compressed and cooled gas is directed to an expander wherein its pressure and its temperature are both reduced by extracting work from the expander. The second flow portion is conducted from the expander to the at least one heat exchanger wherein it serves as a cooling medium therefor. The second flow portion from the at least one heat exchanger is mixed with any saturated vapor from the collector and recirculated to the compressor inlet.
The embodiment just described can be modified to remove gases having a lower boiling point temperature than methane from the feed stock stream. This is often important since the accumulation of these lower boiling point gases such as nitrogen, helium, hydrogen, and the like, in the recirculating stream will eventually "poison" the system so that it cannot handle any new natural gas inlet flow, but can only recirculate the stream of lower boiling point gases.
Removal of the lower boiling point gases is accomplished by altering the vent return path to permit the vent return gas to be burned in an internal combustion type engine, or to be disposed of through the engine exhaust. In the embodiment having this modification, it is preferred that a gas-fueled engine be used. Such an engine can be partially or wholly fueled by the return vent gas.
There are other ways to remove these lower boiling point gases, as will be set forth hereinafter.
Both embodiments just described can be modified to have a second heat exchanger, as will be apparent hereinafter.